| Document and Entity Information | Aug. 12, 2021 |
| Cover [Abstract] | |
| Entity Tax Identification Number | 00-0000000 |
| Amendment Flag | false |
| Entity Central Index Key | 0001805890 |
| Document Type | 8-K |
| Document Period End Date | Aug. 12, 2021 |
| Entity Registrant Name | FUSION PHARMACEUTICALS INC. |
| Entity Incorporation State Country Code | Z4 |
| Entity File Number | 001-39344 |
| Entity Address, Address Line One | 270 Longwood Road South |
| Entity Address, City or Town | Hamilton |
| Entity Address, State or Province | ON |
| Entity Address, Country | CA |
| Entity Address, Postal Zip Code | L8P 0A6 |
| City Area Code | (289) |
| Local Phone Number | 799-0891 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common shares, no par value per share |
| Trading Symbol | FUSN |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
